Optical signals received through conventional optical links are typically distorted by significant amounts of chromatic dispersion (CD) and polarization dependent impairments such as Polarization Mode Dispersion (PMD), polarization angle changes and polarization dependent loss (PDL). Chromatic dispersion (CD) on the order of 30,000 ps/nm, and polarization rotation transients at rates of 105 Hz are commonly encountered. Various methods and systems intended to address some of these limitations are known in the art.
FIG. 1 schematically illustrates a representative coherent optical receiver capable of implementing the methods of Applicant's co-pending U.S. patent application Ser. No. 11/294,613 filed Dec. 6, 2005 and entitled “Polarization Compensation In A Coherent Optical Receiver”; Ser. No. 11/315,342 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Dispersion Impairments”; Ser. No. 11/315,345 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Polarization Impairments”; Ser. No. 11/366,392 filed Mar. 2, 2006 and entitled “Carrier Recovery In A Coherent Optical Receiver”; and Ser. No. 11/423,822 filed Jun. 13, 2006 and entitled “Signal Acquisition In A Coherent Optical Receiver”, the content of all of which are hereby incorporated herein by reference.
As may be seen in FIG. 1, an inbound optical signal is received through an optical link 2, split into orthogonal received polarizations by a Polarization Beam Splitter 4, and then mixed with a Local Oscillator (LO) signal 6 by a conventional 90° optical hybrid 8. The composite optical signals 10 emerging from the optical hybrid 8 are supplied to respective photodetectors 12, which generate corresponding analog electrical signals 14. The photodetector signals 14 are sampled by respective Analog-to-Digital (A/D) converters 16 to yield raw multi-bit digital signals 18 corresponding to In-phase (I) and Quadrature (Q) components of each of the received polarizations.
The resolution of the A/D converters 16 is a balance between performance and cost. It has been found that a resolution of n=5 or 6 bits provides satisfactory performance, at an acceptable cost. The sample rate of the A/D converters 16 is selected to satisfy the Nyquist criterion for the highest anticipated symbol rate of the received optical signal.
From the A/D converter 16 block, the respective n-bit I and Q signals 18 of each received polarization are supplied to a respective dispersion compensator 20, which operates on the raw digital signal(s) 18 to at least partially compensate chromatic dispersion of the received optical signal. Various methods may be used to implement the dispersion compensators 20. For example, a digital Finite Impulse Response (FIR) filter block which applies a predetermined compensation function c□ to the raw signals 18 may be used for this purpose. In some embodiments, the compensation function c□ implemented by each dispersion compensator 20 can be implemented using a respective set of compensation coefficients, which can be adaptively computed by a coefficient calculator 22, for example using the methods described in Applicant's co-pending U.S. patent application Ser. No. 11/328,199 filed Jan. 10, 2006.
The dispersion compensated digital signals 24 appearing at the output of the dispersion compensators 20 are then supplied to a 1:M distribution unit 26, which operates to distribute the signals 24 across M parallel data paths, each of which operates at a lower sample rate (by a factor of M).
In the illustrated embodiment, the distribution unit 26 is implemented as a “burst switch” controlled by a framer 28, to generate successive blocks of samples which can then be routed to each data path. One implementation of a burst switch may, for example, include a multi-port Random Access Memory (RAM).
Within each path, a polarization compensator 30 operates to de-convolve the transmitted I and Q signal components of each polarization from the complex signals 24 output from the dispersion compensators 20. If desired, the polarization compensator 30 may operate as described in Applicant's co-pending U.S. patent application Ser. No. 11/294,613 filed Dec. 6, 2005. The output of the polarization compensator 30 is a pair of multi-bit estimates 32 X′(n) and Y′(n) of the symbols encoded on each transmitted polarization. These symbol estimates 32 X′(n), Y′(n) contain both amplitude and phase information of each transmitted symbol, but also include phase error due to the frequency offset between the Tx and LO frequencies, laser line width and phase noise. In some embodiments, the symbol estimates 32 are 10-bit digital values, comprising 5-bits for each of real and imaginary components of each symbol estimate. The symbol estimates 32 X′(n), Y′(n), appearing at the output of the polarization compensator 30 are then supplied to a carrier recovery block 34 for LO frequency control, symbol detection and data recovery, such as described in Applicant's co-pending U.S. patent application Ser. No. 11/366,392 filed Mar. 2, 2006.
Referring to FIG. 2, the optical signal preferably includes nominally regularly spaced SYNC bursts 36 (which may also be referred to as a framing pattern) embedded within a stream of data symbols 38, as described in Applicant's co-pending U.S. patent application Ser. No. 11/328,199 filed Jan. 10, 2006. Each SYNC burst 36 has a respective predetermined symbol (or, equivalently, bit) sequence on each transmitted polarization. The symbol (bit) sequences of each polarization can be transmitted simultaneously, but this is not essential. In the embodiment of FIG. 2a, two orthogonal bit sequences are used in each SYNC burst 36; each bit sequence being assigned to a respective transmitted polarization. FIG. 2b illustrates an alternative arrangement, in which each of the I and Q components of each transmitted polarization is assigned a respective orthogonal bit sequence.
Framing methods are known for high speed binary signals. For example, U.S. Pat. No. 7,046,700 teaches methods for spectrally invisible framing of a high speed binary signal. However, detection of the frame within the signal presupposes that the binary bit stream has been successfully decoded. On the other hand, signal acquisition in equipment such as a high speed coherent optical receiver requires the identification of the frame or SYNC location, before the binary bit stream has been decoded.
Accordingly, methods and techniques that enable reliable detection of a SYNC burst within a received optical signal, in the presence of moderate to severe impairments remain highly desirable.